The present invention relates to a coated, biaxially oriented film structure having low haze percentage, excellent non-blocking properties, consistently low coefficient of friction (COF) as an unsupported film and as a supported film which is laminated to another film such as glassine.
Polypropylene films are widely used in the packaging industry due to their superior physical properties, such as high tensile modulus and stiffness, excellent optical clarity, and good moisture barrier characteristics, etc. With all its good characteristics, unmodified polypropylene, however, has the disadvantageous properties of a high inherent COF and film-to-film destructive blocking on storage. As claimed by U.S. Pat. No. 3,176,021, COF characteristics of polypropylene and other thermoplastic films can be beneficially modified by the inclusion in the polymer of fatty acid amides. The effectiveness of the amides relies upon their ability to migrate to the surface of the films in order to reduce COF. While amides do improve the COF of the films, the value of the COF is subject to wide variation depending upon the heat history which the film experiences during shipping, storage, and certain converting processes. In addition, the presence of such amides on the film surfaces can adversely affect the film's appearance as manifested by an increase in haze, a decrease in gloss and the presence of streaks. The presence of the amides on the surface can also adversely affect the wettability and adhesion of solvent and water-based inks, coatings, and adhesives.
It is also known to coat polypropylene film with certain fatty acid amides to impart lubricating and antiblocking characteristics as taught in U.S. Pat. No. 4,255,644. However, the application of such coatings by the film manufacturer is not particularly attractive because of the requirement that they be applied as solutions in organic solvents. Health and safety factors dictate against the in-plant utilization of organic solvents in coating processes undertaken during the film manufacture. U.S. Pat. No. 4,302,506 discloses that a polyester film having excellent slip properties and clarity is prepared using a latex coating containing steramideopropyl-dimethyl-.beta.-hydroxyethylammonium nitrate and a crosslinkable acrylic copolymer. Such a coating, however, is not compliant with FDA regulation for food contact applications. Therefore, it is not suitable for food packaging films.
U.S. Pat. Nos. 4,533,509 and 4,618,527 disclose a multi-layer structure comprising a comparatively thick base layer of an optically clear thermoplastic resin and a comparatively thin surface layer containing a finely divided inorganic material in a proportion sufficient to thereby impart antiblock characteristics and decrease film-to-film COF. The resulting film has non-blocking and improved slip characteristics which are stable regarding heat history without any adverse effects on appearance, wetting and adhesion as typically experienced with amide-modified films. However, when laminated to other films, e.g., glassine paper, such structures exhibit significantly higher COF values and do not perform flawlessly on conventional form, fill and seal machines.
U.S. Pat. No. 4,419,410 discloses an oriented multi-layer polypropylene film structure having a surface-modifying agent selected from the group consisting of a slip agent, antistatic agent, antifog agent, antiblock agent and mixtures thereof bloomed on the surface of said polypropylene film structure.
It is an object of the present invention to provide a film having a reduced COF and non-blocking characteristics in comparison with the unmodified film.
It is another object of the present invention to provide a film having non-blocking and improved slip characteristics that are stable with regard to the heat history of the film.
It is yet another object of the present invention to provide a film having non-blocking and improved slip characteristics without the adverse effects on appearance, wetting and adhesion typically experienced with amide-modified films.
It is still another of the present invention to provide a film having non-blocking and improved slip characteristics that are maintained upon lamination of the film to other films, such as glassine.